Applying compliant compliant interfacial layers in thermoelectric devices

ABSTRACT

A thermoelectric power generation technique is disclosed using one or more mechanically compliant and thermally and electrically conductive layers at the thermoelectric material interfaces to accommodate high temperature differentials and stresses induced thereby. The compliant material may be metal foam or metal graphite composite (e.g. using nickel) and is particularly beneficial in high temperature thermoelectric generators employing Zintl thermoelectric materials. The compliant material may be disposed between the thermoelectric segments of the device or between a thermoelectric segment and the hot or cold side interconnect of the device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. Utility patent application Ser.No. 11/770,494, filed Jun. 7, 2013, and entitled “COMPLIANT INTERFACIALLAYERS IN THERMOELECTRIC DEVICES,” by Firdosy et al. (Attorney DocketCIT-5136-P2), incorporated by reference herein, which claims the benefitunder 35 U.S.C. §119(e) of the following U.S. provisional patentapplications, which are incorporated by reference herein:

U.S. Provisional Patent Application No. 61/775,221, filed Mar. 8, 2013,and entitled “COMPLIANT INTERFACIAL LAYERS IN THERMOELECTRIC DEVICES,”by Firdosy et al. (Attorney Docket CIT-6136/CALTP034); and

U.S. Provisional Patent Application No. 61/626,926, filed Jun. 7, 2012,and entitled “NICKEL-GRAPHITE COMPOSITE COMPLIANT INTERFACE AND/OR HOTSHOE MATERIAL,” by Firdosy et al. (Attorney Docket CIT-6225-P).

STATEMENT OF GOVERNMENT RIGHTS

The invention described herein was made in the performance of work undera NASA contract, and is subject to the provisions of Public Law 96-517(35 USC 202) in which the Contractor has elected to retain title.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to thermoelectric devices. Particularly, thisinvention relates to interfacing materials for thermoelectric materialsin thermoelectric power generation devices, particularly those usingZintl, e.g. Yb₁₄MnSb₁₁ (YMS) and skutterudite.

2. Description of the Related Art

Thermoelectric materials exhibit the property of producing an electricvoltage from an applied temperature differential across the material,the so-called thermoelectric effect or Seebeck effect. Accordingly, suchmaterials may be used in thermoelectric devices to generate electricalpower from a temperature differential. Such thermoelectric generatorshave been used to convert heat directly to electrical power forapplications including isolated facilities or space applications.Depending upon the application, the applied heat may be naturallyavailable or generated, e.g. by burning fuel or from a decayingradioisotope.

As mentioned, thermoelectric materials are known to provide a means fordirectly converting heat into electrical energy in a fully solid statedevice. Due to the nature of thermoelectric materials, power generatingdevices require a pairing of two different materials, typicallycomprised of highly doped narrow band gap semiconductors (one with anexcess of n-type charge carriers, the other with an excess of p-typecarriers) connected in a junction.

Prior art thermoelectric devices have featured materials such as silicongermanium, lead telluride, bismuth telluride or other related materials.To achieve greater device efficiency and greater specific power,however, new thermoelectric materials, are required in more complexcombinations. One suitable material is found in the class of Zintlmaterials, particularly the compound p-type semiconductor Yb₁₄MnSb₁₁(YMS), which has been demonstrated to have one of the highest zT valuesat 1000° C., a typical operational temperature of space-basedradioisotope thermoelectric generators (RTGs).

For example, some thermoelectric power generation for deep spaceapplications have employed SiGe thermoelectric materials generatingelectric power using a decaying radioisotope, e.g. plutonium 238, as aheat source, in an RTG. The fuel source and solid state nature of thedevices afford exceptional service life and reliability, paramountconsiderations in space applications which offset the relatively lowefficiency of such devices. Many working RTG devices for spaceapplications have been developed and successfully employed. See e.g.Winter et al., “The Design of a Nuclear Power Supply with a 50 Year LifeExpectancy: The JPL Voyager's SiGe MHW RTG,” IEEE AES Systems Magazine,April 2000, pp. 5-12; and U.S. Pat. No. 3,822,152, issued Jul. 2, 1974to Kot, which are incorporated by reference herein.

Recent focus on renewable energy and increased energy efficiency hasresulted in increased interest in thermoelectric materials and devicesfor applications such as automotive and industrial waste heat recovery.Zintl materials in particular have been studied for thermoelectricapplications. A particular Zintl compound, Yb₁₄MnSb₁₁, has shownexceptional promise for thermoelectric power generation applications.See e.g. Brown et al., “Yb₁₄MnSb₁₁: New High Efficiency ThermoelectricMaterials for Power Generation,” Chem. Mater., 18, 2006, 1873-1877,which is incorporated by reference herein. However, defining theproperties of a particular material are only a first step in thedevelopment of a practical thermoelectric power generation device usingthat material.

SiGe has been well studied as a thermoelectric material as a result ofprevious RTG development. See e.g. , Rowe, “Recent Advanced inSilicon-Germanium Alloy Technology and an Assessment of the Problems ofBuilding the Modules for a Radioisotope Thermoelectric Generator,”Journal of Power Sources, 19 (1987), pp. 247-259; and “Silicon GermaniumThermoelectric Materials and Module Development Program,” ALO (2510)-T1,AEC Research and Development Rep, Cat. UC33, TID 4500, which areincorporated by reference herein. However, although the generalconfigurations of previously developed SiGe thermoelectric powergeneration devices may be applicable, there are differences in thephysical properties of Zintl materials and SiGe that demand newsolutions in the development of a practical thermoelectric powergeneration devices using Zintl materials; the solutions for SiGethermoelectric materials cannot be readily applied to Zintlthermoelectric materials.

To achieve high thermal-to-electric energy conversion efficiency(“Carnot” efficiency) operating across large temperature differentialsis required. When using solid state thermoelectric devices for powergeneration using high grade heat sources, best conversion efficienciesare achieved by combining the highest performance materials in theirrespective optimum operating temperature ranges into multi-stagecascaded or segmented device architectures. Such segmented architectureshave been used primarily for long life thermoelectric generators onboard space science and exploration missions, operating acrosstemperature differentials in excess of 700 K with maximum hot sideoperating temperatures of up to 1273 K.

Next generation high temperature thermoelectric power generating deviceswill employ segmented architectures and will have to reliably withstandthermally induced mechanical stresses produced during componentfabrication, device assembly and operation. Thermoelectric materialshave typically poor mechanical strength, can exhibit brittle behavior,and possess a wide range of coefficient of thermal expansion (CTE)values. As a result, the direct bonding at elevated temperatures ofthese materials to each other to produce segmented leg components isdifficult and often results in localized microcracking at interfaces andmechanical failure due to the stresses that arise from CTE mismatchbetween the various materials. Even in the absence of full mechanicalfailure, the degraded interfaces can lead to increased electrical andthermal resistances which adversely impact conversion efficiency andpower output.

For example, cracking can occur in a segmented p-type thermoelectric legmade of high CTE (e.g. approximatley 18 ppm) Yb₁₄MnSb₁₁ Zintl material(high temperature segment) bonded to an intermediate CTE (e.g.approximately 13 ppm) Ce_(f)Fe_(4-x)Co_(x)Sb₁₂ skutterudite material(low temperature segment) using a direct brazing method onpre-metallized thermoelectric material segments. In one test a largecrack has been observed post fabrication in the Zintl segment, presumedto have occurred as a consequence of the bonding process.

In view of the foregoing, there is a need in the art for apparatuses andmethods for mechanically compliant interface materials, particularly toaccommodate induced stresses from very high temperature gradients, e.g.above 700 K. There is a need for such apparatuses and methods to havehigh thermal and electrical conductivity. In addition, there is aparticular need for such apparatuses and method to operate with Zintlthermoelectric materials such as Yb₁₄MnSb₁₁. There is also a need forsuch apparatuses and methods in Zintl-based thermoelectric devicesoperating at high temperatures, e.g. around or above 1,000 K and up to1273 K. There is a need for such apparatuses and methods to operate forsuch thermoelectric devices in space applications. These and other needsare met by embodiments of the present invention as detailed hereafter.

SUMMARY OF THE INVENTION

A thermoelectric power generation technique is disclosed using one ormore mechanically compliant and thermally and electrically conductivelayers to accommodate high temperature differentials and stressesinduced thereby. The compliant material may be nickel foam or nickelgraphite composite and is particularly beneficial in high temperaturethermoelectric generators employing Zintl thermoelectric materials. Thecompliant material may be disposed between the thermoelectric segmentsof the device or between a thermoelectric segment and the hot or coldside interconnect of the device.

A typical embodiment of the invention comprises a thermoelectric devicehaving a thermoelectric material for generating electrical power fromheat, an adjacent material thermally and electrically coupled to thethermoelectric material, and a compliant metal layer bonded between thethermoelectric material and the adjacent material. The compliant metallayer comprises a metal having a reduced density structure. Typically,the reduced density structure may be 60% or less of the metal as asolid. In some embodiments, the compliant metal layer may be pressed toadjust thermal and electrical conductivity of the layer prior to bondingbetween the thermoelectric material and the adjacent material. Thecompliant metal layer may be bonded by brazing to the thermoelectricmaterial and the adjacent material. The adjacent material may comprise asecond thermoelectric material or any other element of thethermoelectric device requiring thermal and electrical coupling, e.g.the electrodes.

In further embodiments of the invention the thermoelectric material maycomprise Zintl. In some embodiments, the compliant metal layer maycomprise nickel. In this case, the compliant metal layer may comprise anickel foam or a nickel graphite composite.

In a similar manner, a typical method embodiment of the invention ofbonding thermoelectric materials, comprising the steps of bonding acompliant metal layer to a thermoelectric material for generatingelectrical power from heat, and bonding an adjacent material to thecompliant metal layer opposite the thermoelectric material such that thethermoelectric material and the compliant metal layer are thermally andelectrically coupled. The compliant metal layer comprises a metal havinga reduced density structure. The method embodiment of the invention maybe further modified consistent with the apparatus embodiments describedherein.

Another embodiment of the invention may comprise a thermoelectricdevice, including a thermoelectric material means for generatingelectrical power from heat, an adjacent material thermally andelectrically coupled to the thermoelectric material means, and acompliant metal layer means for reducing thermal stresses bonded betweenthe thermoelectric material means and the adjacent material. Thecompliant metal layer means comprises a metal having a reduced densitystructure. This embodiment of the invention may be further modifiedconsistent with the apparatus and method embodiments described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1A is a schematic diagram of an exemplary segmented thermoelectricgenerator employing a thermoelectric material with a compliant metallayer;

FIG. 1B is a schematic diagram of an exemplary cascaded thermoelectricgenerator employing a thermoelectric material with a compliant metallayer,

FIG. 1C is an example enlarged view of a compliant metal layer between athermoelectric material and an adjacent material;

FIG. 2 is plot of example Zintl segment interfacial stresses (as part ofa segmented Zintl/skutterudite leg) for various grades of porous Ni andCu foams;

FIG. 3 is a flowchart of an exemplary method of forming a compliantmetal layer on a thermoelectric material in a thermoelectric device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

1. Overview

As previously mentioned, embodiments of the present invention aredirected to forming compliant materials to accommodate thermally inducedstresses in thermoelectric materials, particularly Zintl thermoelectricmaterials such as Yb₁₁MnSb₁₄, while still delivering the necessarythermal and electrical conductivity. Generally speaking, embodiments ofthe invention are directed to using compliant metal composition layersat material interfaces. The compliant metal composition have a structureof reduced density in order to achieve an adequate level of mechanicalcompliance such that induced thermal stresses in the overall device arealleviated at the interface. A metal, such as nickel, is selected inorder to achieve the thermal and electrical conductivity required at thethermoelectric material interfaces in such devices. The reduced densityof the metal composition can be achieved through different techniques aswill be appreciated by those skilled in the art.

In one example, a the composition may comprise a metal “foam,” e.g. anickel foam. Nickel foam is a known nickel-based material having lowdensity permeable material with numerous applications. A keycharacteristic of such foams is a very high porosity, e.g. typically75-95% of the volume may comprise void spaces. Historically, suchmetallic foams have been employed in a wide variety of applications inheat exchangers, energy absorption, flow diffusion and lightweightoptics.

Another suitable compliant metal composition is a nickel graphitecomposite. In this case, known graphite powder which has been pre-coatedwith nickel (e.g. 60 weight percent nickel) may be hot pressed (e.g. at1000 C under 18,000 psi) to form the compliant metal composition. Anyother known suitable manufacturing process may also be employed whichwill yield a solidified low density composition material of the nickelgraphite powder may also be used.

2. Compliant Interfacial Materials in Thermoelectric Devices

FIG. 1A is a schematic diagram of an exemplary segmented thermoelectricgenerator device 100 employing two thermoelectric material legs 102A,102B. The thermoelectric device 100 employs a thermoelectric material,such as Zintl, for both thermal and electrical conduction. Thethermoelectric material legs 102A, 102B of the thermoelectric device 100generate electrical power directly from the applied thermal gradientbetween the heat collector 108 at one end and the cold shoe 110 at theother end. One thermoelectric material leg 102B acts as an n-typematerial providing excess electrons while the other thermoelectricmaterial leg 102A acts as an p-type material with deficient electrons.

The two thermoelectric material legs 102A, 102B are thermally coupled inparallel between the heat collector 108 and cold shoe 110 butelectrically isolated from one another. Heat is provided to the heatcollector 108 from a coupled heat source 106, e.g. a decayingradioisotope such as plutonium 238 or any other suitable heat sourcecapable of generating temperatures at or above 1000° C. Representativeheat collectors can comprise graphite, nickel, silicon or any othermaterials which can be bonded to the thermoelectric elements and otherelements and which exhibit sufficient thermal conductivity and stabilityat the operating temperature of the device. The cold shoe 110 at theopposing end may include a radiator for rejecting heat to enhance thetemperature differential across the thermoelectric material legs 102A,102B. Electrical power is yielded from an electrical series connectionacross the two thermoelectric material legs 102A, 102B at electrodes114A, 114B. Example electrode materials include copper, nickel and theirrelated alloys. Typically, the electrical power is coupled to a powersystem 112 which may include a regulator and/or battery subsystems asknown in the art.

Typically, a Zintl-based thermoelectric material, e.g. Yb₁₁MnSb₁₄, orskutterudite, may be employed for the p-type thermoelectric material leg102A, while another thermoelectric material, such as silicon germanium,lanthanum telluride or other n-type thermoelectric materials, may beemployed for the n-type leg 102B. However, those skilled in the art willappreciate that other combinations of thermoelectric materials may beemployed for embodiments of the invention. Furthermore, eachthermoelectric material leg 102A, 102B may comprise a combination ofthermoelectric materials.

As previously discussed, the temperatures involved in the manufactureand operation of the device 100 can induce high stresses capable offracturing elements and reducing the efficiency or possibly renderingthe device 100 inoperable. Accordingly, embodiments of the invention canreduce or eliminate such thermally induced stresses through the use ofone of more mechanically compliant metal layers 104A-104F.

The compliant metal layers 104A-104F may be employed within the deviceat any interface of a thermoelectric material and another adjacentmaterial which must be both thermally and electrically conductive. Thus,the compliant metal layers 104A-104F may be used within eitherthermoelectric material leg 102A, 102B (e.g. compliant metal layers104C, 104F) at the interfaces of the electrodes 114A, 114B or theinterfaces of the heat collector 108 (e.g. compliant metal layers 104A,104D). In addition, because many thermoelectric devices may employmultiple different thermoelectric materials within the legs 102A, 102B,the compliant metal layers 104A-104F may be disposed at the interfacebetween two different thermoelectric materials (e.g. compliant metallayers 104B, 104E) arranged in series within the thermoelectric materiallegs 102A, 102B.

In order to be properly compliant, the metal layers 104A-104F eachcomprise a metal having a reduced density structure (compared to themetal as a solid). For example, nickel (or other metals such as copper)may be used in a porous form known as nickel “foam.” The metal structureis very porous having a lower density than solid nickel. In someapplications the reduced density structure may be 60% or less of themetal (e.g. nickel) as a solid. Another suitable reduced density metalstructure may be produced by sintering powders. For example, a nickelgraphite composite may be formed from graphite powder which has beenpre-coated with nickel (e.g. 60 weight percent nickel) and hot pressed(e.g. at 1000 C under 18,000 psi) to form the compliant metal structure.The lowered density of the metal structure provides the compliance toabsorb thermally induced stresses arising from the expansion of theother materials in the device. Bonding of the compliant metal layers104A-104F may be achieved through diffusion bonding or brazing or anyother suitable technique known to provide a conductive joint with thecompliant metal material. (Correct?)

It should also be noted that design of particular interfaces using thecompliant metal layers may be readily adjusted through a simple coldworking process applied to the layers. Recognizing that the densityvaries inversely with the conductivity (thermal and electrical) for agiven low density metal structure, by pressing the compliant materiallayers, the conductivity may be increased at the cost of some density(i.e. some compliance). In this way, compliant layers may be “tuned” forthe optimum balance of properties in a particular application as will bereadily appreciated by those skilled in the art.

FIG. 1B is a schematic diagram of an exemplary cascaded thermoelectricgenerator device 150 employing a thermoelectric material with acompliant metal layer. This device 150 includes two stages in seriesacross the applied overall thermal gradient. Each stage essentiallyoperates in the same manner as the single stage device of FIG. 1A andthe prior description is generally applicable as will be understood bythose skilled in the art. However, the interface between the stagesincludes an upper electrically non-conductive element 122 toelectrically isolate the stages from one another. (Correct?) Both theseinterface elements are thermally conductive, however. For each stage,the compliant metal layers 104A-104L may be employed between athermoelectric material and an adjacent material thermally andelectrically coupled thereto; e.g. at the electrodes 114A-114D(compliant metal layers 104C, 104F, 104G, 104H), at the heatercollectors 108A, 108B (compliant metal layers 104A, 104D, 104I, 104J),and between different thermoelectric materials with the legs 102A-102D(compliant metal layers 104B, 104E, 104K, 104L). Electrical power iscoupled to a power system 112 (which may be two separate systems foreach stage) from the electrodes 114A-114D which may include a regulatorand/or battery subsystems as known in the art.

FIG. 1C is an example enlarged view of a compliant metal layer 104between a thermoelectric material 118 and an adjacent material 120. Notethe thermoelectric material 118 and adjacent material 120 may be any ofthe applicable elements previously described—the upper and lowerpositions as shown may be switched. Bonding between the compliant metallayer 104 and both the adjacent material 120 and the thermoelectricmaterial 118 may be achieved through by brazing leaving a thin layer116A, 116B of brazing material therebetween to provide a conductivejoint between the compliant metal layer 104 and the other materials. Thebrazing material may be Cusil-ABA (comprising silver, titanium, andcopper), for example, or any other suitable composition.

It should be noted that the thermoelectric devices 100, 150 depicted inFIGS. 1A & 1B are not to scale and present only example thermoelectricpower generation devices. The thermoelectric devices 100, 150 are justexample configurations of embodiments of the invention. Those skilled inthe art will appreciate that the general configurations of previouslydeveloped thermoelectric power generation devices, e.g. SiGe and otherRTGs, as well as newer Zintl thermoelectric devices may be employed withcompliant metal layers. In addition, the thermoelectric material legs102A, 102B may also include other materials, e.g. to facilitateelectrical connection to the power system 112 and electrical isolation,e.g. graphite barriers may be employed in the element stack. The heatingelement 106 need not be directly adjacent to the heat collector 108 butmay only be radiatively coupled to the heat collector 108 instead.

It should also be noted that embodiments of the invention are notrestricted to thermoelectric power generation devices, but may beapplied to any application (e.g., possibly thermoelectric coolers andheaters as well) employing a thermoelectric material which may undergohigh thermally induced stresses. In addition, embodiments of theinvention are not restricted to space applications in a vacuumenvironment. Embodiments of the invention may be employed with otherenvironments surrounding the thermoelectric material. For example, someapplications may utilize an Ar environment surrounding thethermoelectric material.

3. Forming Compliant Metal Interfaces in Thermoelectric Device

Embodiments of the invention also encompass a method of formingcompliant metal interfaces in a thermoelectric device. Detailed finiteelement analyses of thermally induced mechanical stress duringthermoelectric component fabrication and segmented leg assembly canindicate that thin layers of low modulus porous metal foams can yieldlarge reductions in tensile and/or compressive stresses at interfaceswhile maintaining acceptable electrical and thermal conductancecharacteristics. Free-standing segmented thermoelectric legs may befabricated by bonding a thin nickel foam layer between the hightemperature, high CTE Zintl segment and a lower temperature, lower CTEskutterudite segments. The elements may be brazed using a layer ofCusil-ABA braze in-between the nickel foam and the metallized Zintl andskutterudite segments. For bonding, the segments may be heated to 630°C. and held for 20 minutes with approximately 50 psi of compressive loadunder high vacuum. This bonding technique may be readily applied toother thermoelectric materials into segmented thermoelectric materiallegs as will be appreciated by those skilled in the art.

Alternately, graphite powders that have been pre-coated with 60 weightpercent nickel (e.g. from Novamet Specialty Products Corp.) may be hotpressed (at 1000° C. under 18000 psi) to form compliant metal layersfrom the nickel-graphite composite material. A free standingthermoelectric segmented leg may be fabricated by bonding the compliantpad layer between the high temperature (p-Zintl) and low temperature(p-SKD) thermoelectric segments brazed using a layer of Cusil-ABA brazein-between the nickel-graphite composite and the metallized Zintl andskutterudite segments The segments may be heated to 630° C. and held for20 minutes with approximately 50 psi of compressive load under highvacuum. This bonding technique may be readily applied to otherthermoelectric materials into segmented thermoelectric material legs aswill be appreciated by those skilled in the art.

FIG. 2 is plot of example Zintl segment interfacial stresses (as part ofa segmented Zintl/skutterudite leg) for various grades of porous Ni andCu foams. As shown by the plots appreciable benefits arise at relativedensities below 60%, with large reductions in tensile stress predictedfor highly porous foams (i.e. less than 30% of relative density). It isexpected that similar plots may be readily developed for other suitablematerials such as nickel graphite composite.

FIG. 3 is a flowchart of an exemplary method 300 of forming a compliantmetal layer on a thermoelectric material in a thermoelectric device. Themethod 300 begins with an operation 302 of bonding a compliant metallayer, comprising a metal having a reduced density structure, to athermoelectric material for generating electrical power from heat. Inoperation 304, an adjacent material is bonded to the compliant metallayer opposite the thermoelectric material such that the thermoelectricmaterial and the compliant metal layer are thermally and electricallycoupled. The method 300 may be further modified to include the optionaloperation 306 of pressing the compliant metal layer to adjust thermaland electrical conductivity of the layer prior to bonding between thethermoelectric material and the adjacent material.

Embodiments of the invention comprise insertion of a high electrical andthermal conductivity mechanically compliant nickel foam ornickel-graphite composite layer between the low and high temperaturesegments to relieve thermomechanical stresses during device fabricationand operation. These materials can be used as a stress relieving layerbetween the thermoelectric segments and/or between a thermoelectricsegment and a hot or cold side interconnect material. (Thenickel-graphite material can also be used as a compliant hot shoematerial.) A comparison of the electrical resistivity, coefficient ofthermal expansion (CTE) and Young's modulus is shown below for nickel,nickel foam (NTR#48519) and the nickel-graphite composite.

TABLE 1 Relevant property comparison for Ni, Ni foam and Ni-C compositematerials Nickel Nickel Nickel (100% foam (90% (60%)- graphite Propertydense) porosity) Composite Electrical resistivity 6.85E−8 2E−6 3E−7(Ohm-m) Average CTE (ppm/° C.) 16.6 16.6 (estimated) 14.4 (RT-500° C.)Young's Modulus 207   2 (estimated) 4-35 (Gpa)

As described, compliant metal layers for thermoelectric devices may beformed from low elastic modulus metal foam, a porous material thatreduces the thermo-mechanical stresses encountered in the constructionof high efficiency, high temperature thermoelectric device. In addition,embodiments of the invention may also employ the use of a compositematerial that reduces the thermo-mechanical stresses encountered in theconstruction of high efficiency, high temperature thermoelectricdevices. In use, the metal foam or composite composition or and thus CTEcan be adjusted to minimize mismatch with the parts to be joined, andcan easily be brazed to the metallized surfaces of the thermoelectricmaterials segments, while maintaining the desired electrical and thermalproperties essential for efficient device operation. The modulus,stiffness, electrical and thermal conductances of the materials can becontrolled through the type and amount of porosity, its chemicalcomposition and pressing (cold working) to fine tune. This improvementoffers a versatile technique to joining various thermoelectric materialsinto segmented device architectures and maximizes energy conversionefficiency.

This concludes the description including the preferred embodiments ofthe present invention. The foregoing description including the preferredembodiment of the invention has been presented for the purposes ofillustration and description. It is not intended to be exhaustive or tolimit the invention to the precise forms disclosed. Many modificationsand variations are possible within the scope of the foregoing teachings.Additional variations of the present invention may be devised withoutdeparting from the inventive concept as set forth in the followingclaims.

What is claimed is:
 1. A method of bonding thermoelectric materials, comprising the steps of bonding a compliant metal layer to a thermoelectric material for generating electrical power from heat; and bonding an adjacent material to the compliant metal layer opposite the thermoelectric material such that the thermoelectric material and the compliant metal layer are thermally and electrically coupled; wherein the compliant metal layer comprises a metal having a reduced density structure.
 2. The method of claim 1, wherein, wherein the reduced density structure is 60% or less of the metal as a solid.
 3. The method of claim 1, further comprising pressing the compliant metal layer to adjust thermal and electrical conductivity of the layer prior to bonding between the thermoelectric material and the adjacent material.
 4. The method of claim 1, wherein the compliant metal layer is bonded by brazing to the thermoelectric material and the adjacent material.
 5. The method of claim 1, wherein the adjacent material comprises a second thermoelectric material.
 6. The method of claim 1, wherein the thermoelectric material comprises Zintl.
 7. The method of claim 1, wherein the compliant metal layer comprises nickel.
 8. The method of claim 7, wherein the compliant metal layer comprises nickel foam.
 9. The method of claim 7, wherein the compliant metal layer comprises nickel graphite composite.
 10. The method of claim 7, wherein the compliant metal layer comprises nickel with less than 60% of the nickel as a solid.
 11. The method of claim 10, wherein the compliant metal layer comprises less than 30% of the nickel as a solid.
 12. The method of claim 1, wherein the compliant metal layer comprises copper.
 13. The method of claim 12, wherein the compliant metal layer comprises nickel with less than 60% of the nickel as a solid.
 14. The method of claim 13, wherein the compliant metal layer comprises less than 30% of the copper as a solid. 